In device fabrication, optical lithography, such as projection printing using steppers and scanners, is a technique for transferring a desired pattern from a mask onto the surface of a semiconductor wafer. Such technique employs an exposure source to image the pattern through an optical system. The pattern corresponds to circuit features that comprise various parts of an electronic device. The continuing demand for further miniaturization of devices has facilitated the use of exposure sources with shorter wavelengths in order to achieve smaller image resolution. For example, projection printers operating in the DUV region (248 nm or shorter) have been able to image linewidths of about 0.25 .mu.m. However, DUV radiation, at least at the intensity levels which are desirable for commercial applications, tends to cause damage to the optical elements in the projection printers.
In conventional DUV projection printers, the optical system comprises mostly or all transmissive elements. One of the few materials for high precision optics which transmits DUV radiation is fused silica. Rothschild et al., Excimer Laser Projection Lithography: Optical Considerations, Microelectronic Engineering Vol. 9, pages 27-29 (1989), which is herein incorporated by reference for all purposes. As such, it has become the material of choice for use in the manufacturing of optical elements for DUV printers. To achieve the desired image resolution, i.e., image resolution similar to the wavelength of the exposure source, the optical system is diffraction limited. In diffraction limited systems, the specifications of the lenses are well below diffraction limited and balanced in the overall system for small net wavefront aberration. For example, the root mean square (RMS) wavefront error for a typical optical system is about 1/10.sup.th of the wavelength of the exposure source. Even stricter requirements imposed on the individual optical elements make them very sensitive to phenomenon which would alter their behavior.
The predominant exposure sources used in DUV projection printers are excimer lasers, such as the KrF (248 nm) and ArF (193 nm). This is due to the fact that excimer lasers can generate adequate power output (about 2-20 W) necessary to satisfy manufacturing throughput demands. In addition, the power output of excimer lasers is relatively stable, which is important for dose control. For example, fluctuations in output amplitude of a typical excimer laser is about .ltoreq.3%.
The power output of the excimer lasers is generated by short pulses. "Short pulses" are pulses of a frequency F each having a pulse width (.tau.) that is significantly less than the time gap between each pulse (1/F). As known in the art, the shape of excimer pulses varies with the type of excimer laser being used and therefore, may be difficult to clearly define the pulse width of an excimer pulse. For convenience, the pulse width, as used herein, begins where the energy of the pulse can be detected by, for example, attenuators or silicon photo-diodes and ends at about the point where 99% of the pulse energy has been detected. For current commercially available excimer lasers, the pulse width is about 10-30 nsec and F is about 200-500 Hz.
Each output pulse or shot from an excimer laser is typically about 10-50 mJ. Studies have shown that fused silica, when exposed to such intensity levels at 248 or shorter, is subjected to phenomenon known as laser-induced damage. See Rothschild et al., Excimer Laser Projection Lithography: Optical Considerations, Microelectronic Engineering, Vol. 9, pages 27-29 (1989), which is already incorporated by reference for all purposes. The major modes of laser-induced damage in fused silica are color center formation and optical compaction. The formation of E' color centers leads to increased optical absorption at the exposure source wavelength. Optical compaction alters the lens dimensionally and increases the index of refraction in the compacted area, thus causing wavefront aberrations such as 1) instantaneous changes in the wavefront properties of the lens as a pulse is transmitted, 2) accumulated changes in the wavefront or overall transmission efficiency of the lens, and 3) thermal degradation of the lens wavefront due to absorption of a larger fraction of power. As such, laser-induced damage adversely affects the characteristics of the optical system, thus degrading the performance of projection printers.
The initial mechanisms causing the damage have been identified as the multi-photon interactions in the material activated by extremely high momentary intensity levels. The rate of multi-photon absorption laser-induced damage often has a quadratic dependence to pulse intensity in the practical regime of intensities that might be used. Furthermore, laser-induced damage to the optics is even more severe as the wavelength of the exposure source becomes shorter because fused silica is much more absorptive at shorter wavelengths. For example, the rate of laser-induced damage at 193 nm for ArF laser is much greater than that at 248 nm for KrF laser. See Rothschild et al., Excimer Laser Projection Lithography: Optical Considerations, Microelectronic Engineering Vol. 9, pages 27-29 (1989).
In conventional scanners and steppers, projection lenses with reduction powers may have some elements near the wafer. These elements are exposed to the greatest intensity of energy from each pulse and therefore are most prone to laser induced damage. Moreover, the projection lens is one of the more critical and more expensive elements of the system. From the above description, it is therefore desirable to reduce the rate of damage to the optical system of projection printers and, in particular, to the projection lens of the optical system caused by exposure to DUV energy pulses, particularly as shorter wavelengths, photo resists requiring larger doses, or longer useful lens life becomes desirable.